quota: protect Q_GETFMT by dqonoff_mutex

dqptr_sem will go away. Protect Q_GETFMT quotactl by
dqonoff_mutex instead. This is also enough to make sure
quota info will not go away while we are looking at it.
Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/quota.c

@@ -79,13 +79,13 @@ static int quota_getfmt(struct super_block *sb, int type, void __user *addr)
 {
 	__u32 fmt;
 
-	down_read(&sb_dqopt(sb)->dqptr_sem);
+	mutex_lock(&sb_dqopt(sb)->dqonoff_mutex);
 	if (!sb_has_quota_active(sb, type)) {
-		up_read(&sb_dqopt(sb)->dqptr_sem);
+		mutex_unlock(&sb_dqopt(sb)->dqonoff_mutex);
 		return -ESRCH;
 	}
 	fmt = sb_dqopt(sb)->info[type].dqi_format->qf_fmt_id;
-	up_read(&sb_dqopt(sb)->dqptr_sem);
+	mutex_unlock(&sb_dqopt(sb)->dqonoff_mutex);
 	if (copy_to_user(addr, &fmt, sizeof(fmt)))
 		return -EFAULT;
 	return 0;